Polyurethane foams are used in many sectors, such as furniture, mattresses, transport, building and industrial insulation. To meet the high levels of flame resistance required of materials to be used inter alia for the automotive, rail or aircraft interior and also in building insulation, polyurethane foams generally have to be enhanced with flame retardants. A multiplicity of different flame retardants are already known and commercially available for this. However, there are often appreciable technical issues and/or toxicological concerns surrounding their use.
When, for instance, solid flame retardants are used, e.g. melamine, ammonium polyphosphate or ammonium sulphate, sedimentation or aggregation gives rise to metering problems which often necessitate technical modifications to the foaming equipment, i.e. inconvenient revamping and rejigging.
Commonly used chloroalkyl phosphates tris(chloroethyl) phosphate, tris(chloroisopropyl) phosphate and tris(dichloroisopropyl) phosphate are readily meterable liquids. However, a recent but increasingly common requirement of open-cell flexible polyurethane foam systems for the automotive interior is that the gaseous emissions (volatile organic compounds, VOCs) and especially the condensable emissions (fogging) from these foams shall not exceed low limits. The liquids referred to above no longer meet these requirements owing to their excessive volatility.
Fogging refers to the undesired condensation of evolved volatile constituents from the motor vehicle interior on glass panes, in particular on the windscreen. This phenomenon is quantifiable according to DIN 75 201 B. The automotive industry typically requires that the fogging condensate as determined by the DIN 75201 B method shall be less than 1 mg.
Furthermore, halogen-free flame retardants are preferred from ecotoxicological aspects and also by reason of ameliorated fire side-effects regarding smoke gas density and smoke gas toxicity. Halogen-free flame retardants may also be of particular interest for performance reasons. For instance, severe corrosion is observed on the plant components used for flame lamination of polyurethane foams when halogenated flame retardants are used. This is attributable to the emissions of hydrohalic acid which arise during the flame lamination of halogen-containing polyurethane foams.
Flame lamination refers to a process for bonding textiles and foams together wherein one side of a foam sheet is incipiently melted by means of a flame and immediately thereafter pressed together with a textile web.
The automotive and furniture industries are increasingly demanding the use of flame retardants which, especially in open-cell flexible polyurethane foams, cause a very low level of scorch. Scorch, or core discolouration, refers to the undesirable browning in polyurethane foams during manufacture, caused by thermal and oxidative degradation of the polyurethane foam in the presence of air. Core discolouration is observed in particular in the industrial manufacture of large polyurethane foam buns, since this is where the unfavourable surface/volume ratio means that the temperature in the core of the bun remains at an elevated level for longer. On the laboratory scale, core discolouration is quantifiable by the microwave method described in U.S. Pat. No. 4,131,660.
Flame retardants can have an appreciable adverse effect on the core discolouration of polyurethane foams. An addition of chloroalkyl phosphates, for example tris(dichloroisopropyl) phosphate, as flame retardant leads to an appreciable increase in core discolouration, or scorch. Brominated diphenyl ethers, dialkyl tetrabromophthalates and aryl phosphates are low-scorch flame retardants. Aryl phosphates are the flame retardants of choice to provide the combination of low scorch and freedom from halogen.
Aryl phosphates such as triphenyl phosphate (cf. for instance EP 0 170 206 A1) or diphenyl cresyl phosphate (cf. for instance EP 0 308 733 B1) are readily available and make for efficacious flame retardants when used in polyurethane foams. However, triphenyl phosphate has the serious disadvantage of being harmful for aquatic organisms. This applies not just to triphenyl phosphate itself, but also to many commercialized aryl phosphate mixtures with triphenyl phosphate.
A further problem is that the conditions of polyurethane synthesis or the further product lifecycle of the foam may result in a minimal release of phenols. Aryl phosphates are therefore counted among the so-called phenol-formers. Phenol-formers are capable of causing a measurable content of substituted or unsubstituted phenols in a product even when the phenols in question were themselves not even used in the manufacture of the product. Examples of phenol-formers are the phenyl and alkylphenyl esters of organic and inorganic acids. Since the presence of phenol-formers in consumer applications, for example in the automotive sector, is frequently no longer accepted for reasons of product safety, there is a need for equivalent replacements.
Rising expectations of product safety are therefore driving the search for alternatives to triphenyl phosphate-containing flame retardants in polyurethane foams.